The present invention relates to optical time domain reflectometry and more specifically to an optical time domain reflectometer having pre and post front panel connector fiber testing capabilities.
Optical time domain reflectometers, OTDR, are used for testing optical transmission lines, such as fiber optic cables and the like, for discontinuities and faults, which affect the quality of optical signals transmitted through the fiber. In testing an optical fiber with an OTDR, the fiber under test is connected to a front panel connector and optical pulses from a laser are launched into the fiber. During the interval between the pulses, backscattered light from the fiber produced by the Rayleigh effect is directed to a photosensitive detector, such as a avalanche photodiode or the like, in the optical receiver of the OTDR. The detector converts the backscattered light signal into an electrical signal, which is amplified, sampled and displayed on an output device.
Improper alignment of the fiber under test in the front panel connector significantly reduces the amount of optical power launched into the fiber and the amount of Rayleigh backscattering returning from the fiber. One method of testing the front panel connection is to compare the optical power from the fiber with a reference value. If the power falls below the reference value, then the connector is considered faulty. However, this type of test can be inaccurate. For example, low optical power output from the laser would produce the same results as a faulting front panel connection.
Another method is used in the Model MW910A OTDR manufactured by Hitachi, Corp., Tokyo, Japan. The OTDR is provided with a light emitting diode, LED, output having a known power level at the back of the instrument. The output from the LED is coupled to the front panel connector of the OTDR via a fiber optic jumper cable. The LED input is displayed on the OTDR and a determination is made as to the quality of the front panel connection. This test, however, will not guarantee that the fiber under test connection will not be faulty.
Many OTDR's use optical modulators, such as a Bragg Cell, as optical couplers for selectively connecting the laser output to the fiber under test and for coupling the backscattered light from the fiber to the optical receiver. When the Bragg Cell switched to the laser output position, the optical receiver is blocked from receiving the backscattered light from the fiber. This characteristic of the Bragg Cell is useful in masking the high amplitude return reflections from the fiber under test caused by fiber splices and connectors, which overdrive the optical detector producing an output electrical pulse that has an approximately exponential decaying trailing edge due to the storage time of the detector. The detector storage effect produces a characteristic detector diffusion tail in the displayed reflected signal. By blocking the high amplitude return reflections, events that would be hidden by the detector tail may be examined. However, using the Bragg Cell as the optical coupler prevents the acquisition of data on the reflection associated with the front panel connector due to the time delay caused by the Bragg Cell having to stay in the laser output position until the transmitted pulse has passed through the cell. This time delay masks the optical receiver from the front panel connector reflection and a portion of the backscatter from the fiber under test.
What is needed is an OTDR that establishes a pre-front panel connector reference level for accurately determining the status of the transmitting and receiving circuitry in the OTDR and the quality of the front panel connection and can acquire front panel connector reflection data when using a Bragg Cell as an optical coupler.